Composite Elements

ABSTRACT

Composite elements have the following layer structure: (i) from 2 to 20 mm of metal, (ii) from 10 to 300 mm of plastic, and (iii) from 2 to 20 mm of metal, where (i) and/or (iii) have an opening which may, if desired, be sealable.

The invention relates to composite elements which have the followinglayer structure:

-   -   (i) from 2 to 20 mm of metal,    -   (ii) from 10 to 300 mm of plastic, preferably compact plastics,        preferably polyisocyanate polyaddition products, particularly        preferably polyurethanes, and    -   (iii) from 2 to 20 mm of metal,        where (i) and/or (iii) have an opening which may, if desired, be        sealable. For the purposes of the present invention, the term        “opening” implies that (i) and/or (iii) do not seal off (ii)        from the surroundings of that side of (i) and/or (iii) which        faces away from (ii).

The invention further relates to the use of open or closed valves incomposite elements of this type, and also to the use of the compositeelements.

The structural components used in the design of ships, for example hullsand cargo hold covers, or of bridges or multistorey buildings, have tobe able to withstand considerable stresses from external forces. Due tothese requirements structural components of this type are usuallycomposed of metal plates or metal supports, strengthened by appropriategeometry or suitable struts. Due to increased safety standards, tankerhulls therefore are usually composed of an inner and an outer hull, eachhull being built up from steel plates of 15 mm thickness, connected toone another via steel struts about 2 m in length. Since these steelplates are exposed to considerable forces, both the outer and the innersteel shells are reinforced by welded-on reinforcing elements.Disadvantages of these traditional structural components are both theconsiderable amounts of steel required and the time-consuming andlabor-intensive method of manufacture. In addition, structuralcomponents of this type have considerable weight, reducing the tonnageof the ships and increasing fuel usage. Traditional structuralcomponents of this type based on steel also require heavy maintenance,since both the outer surface and the surfaces of the steel componentsbetween the outer and the inner shells regularly have to be protectedagainst corrosion.

It is an object of the present invention, therefore, to developstructural components which withstand large loads exerted by externalforces and can be used in the construction of ships, of bridges or ofmultistorey buildings, for example. The structural components to bedeveloped, also called composite elements, are intended to serve as areplacement for known designs based on steel, and in particular to haveadvantages interms of their weight, production process and maintenancerequirement. The composite elements are in particular intended to havecontrollable behaviour which is retained at high temperatures.

Composite elements which are composed of a metal/plastic/metal compositemay be used as an alternative to the designs in steel described in theintroduction. In the case of these composite elements it would beconceivable that if the temperature between the metal layers wereextremely high, an increased pressure could be built up between theselayers. This excess pressure could lead to cracking of the metal layers,and this would be attended by a loss of loadbearing capability of theentire composite element, and any sudden release of pressure wouldcreate a particular risk. A particular object was therefore to preventthe occurrence of any possible risks which would attend a build-up ofpressure in a metal/plastic/metal composite.

We have found that this object is achieved by the composite elementsdescribed at the outset.

At extremely high temperatures it is possible in some cases for there tobe decomposition of the plastic between the metal layers, followed byevaporation creating pressure between the metal layers. This isparticularly the case if the plastic layer (ii) has been completelysurrounded by (i) and (iii). According to the invention any excesspressure of this type between the metal layers (i) and (iii) is releasedvia at least one opening in (i) and/or (iii). This opening may bepresent in a sealed or open or sealable form, and it is preferable forthe closed opening to open automatically as a result of an externaleffect, such as a temperature and/or pressure rise. The opening may havebeen filled with a material, for example, or provided with a seal in theform of a cap, so that if there is excess pressure in (ii) and/or thereis a temperature rise, the opening opens. Examples of materials whichreversibly seal the opening are metals with a melting point below 450°C., e.g. lead and/or tin, preferably lead, or else organic compounds orplastics, preferably with a melting point below 250° C. An advantage ofsealing the opening with the materials mentioned, hereinafter alsotermed filling materials, for example by filling or covering theopening, is that although (ii) is protected from the environment outsidethe composite element, for example water, air or chemicals, any build-upof pressure in (ii), in particular as a result of a temperature rise, israpidly released. The filling material may be introduced into theopening in solid, softened or molten form, for example simply by beingplaced there or by being inserted manually or mechanically, for exampleusing a hammer.

The opening may be composed of a hole in (i) and/or (iii). It is alsopossible to incorporate the opening into (i) and/or (iii) by way of anelement, indicated in the drawings as (ix), for example one made fromthe material used for (i) and/or (iii), and having the opening. Thisincorporation of the element, for example of the valve, which may, ifdesired, have been closed, for example by the filler, may take placeprior to, during or after the production of (ii) between (i) and (iii).The element may be secured by screwing or bolting, welding and/oradhesive bonding, for example to the as yet incompletely reactedreaction system for producing (ii). Appropriate openings and elementsare shown in FIGS. 3, 4, 5 and 6, in which for simplicity the secondmetal layer has not been depicted.

It is preferable for the elements to have flush joints with the surfaceof (i) and/or (iii) on the surface facing away from (ii), i.e. thesurface of (i) and/or (iii) is level with the element incorporated, andthe element does not project therefrom.

The opening preferably has a diameter of from 0.5 to 100 mm,particularly preferably from 1 to 10 mm, very particularly preferablyfrom 2 to 8 mm.

Preference is therefore given to composite elements whose openingaccording to the invention has been closed and opens when the pressuredifference between (ii) and the other, outward-facing, side of (i)and/or (iii), i.e. the region outside the composite element, is at least10 bar, where the higher pressure is present in (ii), and/or opens whenthe temperature is above 250° C., preferably above 450° C.

The opening is preferably a valve which has particularly preferably beenconnected to (i) and/or (iii) by a method using screw threads. Ascrewed-on element of this type, shown by way of example in FIGS. 4 and5, preferably has a diameter of from 5 to 150 mm, particularlypreferably from 15 to 100 mm.

FIG. 1 gives a diagram of the novel composite elements. (i) and (iii) inthe figure are the appropriate metal layers and (ii) is the plastic,which preferably adheres both to (i) and to (iii). The opening isindicated by (iv). FIGS. 3, 4, 5 and 6 are diagrams of embodiments ofthe opening, for example in the form of elements which have an openingand have been incorporated by a method using screw threads.

The figures given in the figures are dimensions in mm.

The composite elements may have been arranged in the form of adouble-walled element of a ship, as shown in FIG. 2. The opening isindicated by (iv). The two composite elements have been indicated by(vi) and (vii). The composite elements have been connected by crossstruts indicated by (viii).

The opening may preferably have been connected to one end of a pipe (x)which extends into (ii) and whose other end is at a distance of from 0.5to 9.5 mm from the surface in contact with (ii) of the metal layer whichdoes not have the opening connected to the pipe. An arrangement of thistype is shown in FIGS. 7 and 8. The figure shows the pipe, the openinterior of which has been connected to the opening. The other end ofthe pipe extends as far as the appropriate distance from the surface ofthe other metal layer. This has the advantage that gas which is formedon one side of the composite element, for example by intense heating asa result of a fire, is removed at that side of the composite elementwhich faces away from the fire. This prevents the fire from igniting thegas.

Preference is therefore given to the use of open or closed valves incomposite elements which have the following layer structure:

-   -   (i) from 2 to 20 mm of metal,    -   (ii) from 10 to 300 mm of plastic, and    -   (iii) from 2 to 20 mm of metal,        for reducing a pressure difference between (ii) and the other        outward-facing side of (i) and/or (iii), where the higher        pressure is present in (ii), and the use of open or closed        valves in composite elements which have the following layer        structure:    -   (i) from 2 to 20 mm of metal,    -   (ii) from 10 to 300 mm of plastic, and    -   (iii) from 2 to 20 mm of metal, for dissipating gases from (ii).

The opening in the figures may, as described above, have been sealed.Even if this is not indicated in detail in the drawings, therefore, afilling material may be present in each opening, or the opening may havebeen sealed by a cap, as described at the outset.

In addition to the abovementioned advantages, the novel compositeelements have excellent mechanical properties.

The novel composite elements may be manufactured by producing, between(i) and (iii), for example, polyisocyanate polyaddition products (ii),usually polyurethanes, which may, if desired, have urea structuresand/or isocyanurate structures, by way of reacting (a) isocyanates with(b) compounds reactive toward isocyanates, preferably in the presence offrom 1 to 50% by volume, based on the volume of the polyisocyanatepolyaddition products, of at least one gas (c), and also, if desired, of(d) catalysts and/or (e) auxiliaries and/or additives, where thesepreferably adhere to (i) and (iii).

Despite the preferred use of (c) the polyisocyanate polyadditionproducts may be termed compact products, since no network of gas-filledcells is formed.

The reaction is preferably carried out in a closed mold, for exampleusing (i) and (iii) as outer layers, so that when the mold is filled (i)and (iii) are in the mold together with the starting components forproducing (ii), and the mold is sealed when all of the startingcomponents have been introduced. Once the starting components have beenreacted to produce (ii) the composite element may be demolded. The usualmethods and materials may be used for the preferred lateral sealing-offof the space between (i) and (iii).

It is preferable to sand-blast those surfaces of (i) and/or (iii) towhich (ii) adheres once the composite elements have been produced. Usualprocesses may be used for this sand-blasting. For example, the usualsand may be used to sand-blast the surfaces at high pressure, thus, forexample, cleaning and roughening the surfaces. Suitable equipment for atreatment of this type is commercially available.

This treatment of those surfaces of (i) and (iii) which are in contactwith (ii) once (a) has been reacted with (b) in the presence of (c) andalso, if desired, (d) and/or (e) can give markedly improved adhesion of(ii) to (i) and (iii). The sand-blasting is preferably carried outdirectly prior to the introduction of the components used for producing(ii) into the space between (i) and (iii).

To produce the composite elements, for example after the preferredtreatment of the surfaces of (i) and (iii), these layers are preferablyfixed in a suitable arrangement, for example parallel to one another.The distance selected is usually such that the space between (i) and(iii) has a thickness of from 10 to 300 mm. (i) and (iii) may be fixedby way of spacers, for example. It is preferable for the edges of theintervening space to be sealed off in a way which allows the spacebetween (i) and (iii) to be charged with (a), (b) and (c), and also, ifdesired, (d) and/or (e), but prevents these components from escaping.For the sealing-off use may be made of the usual plastics films or metalfilms and/or metal plates, and these may also serve as spacers.

The layers (i) and (iii) used may preferably be the usual metal plates,such as steel plates, with the thicknesses according to the invention.

When the space between (i) and (iii) is filled, (i) and (iii) may bevertical or horizontal.

The usual conveying equipment, such as high- or low-pressure machinery,preferably high-pressure machinery, may be used to charge the spacebetween (i) and (iii), preferably continuously, with (a), (b) andpreferably (c) and, if used, (d) and/or (e).

The conveying rate may be varied as a function of the volume to befilled. In order to ensure uniform through-curing of (ii), the conveyingrate and conveying equipment selected is such that the space to befilled can be filled within a period of from 0.5 to 20 min, preferablyfrom 1.5 to 7 min, with the components for producing (ii).

The layers (i) and (iii) used are usually plates and may be the usualmetals, such as iron, conventional steel, any type of refined steel,aluminum and/or copper.

When producing the novel composite elements, either (i) or else (ii) maybe used in coated form, for example primed, otherwise surface-coatedand/or coated with conventional plastics. (i) and (iii) are preferablyused uncoated, and particularly preferably, for example, after cleaningby conventional sand-blasting.

The preparation of the polyisocyanate polyaddition products (ii),usually polyurethane products and, if desired, polyisocyanurateproducts, in particular polyurethane elastomers, by reacting (a)isocyanates with (b) compounds reactive toward isocyanates, preferablyin the presence of (c) and also, if desired, of (d) catalysts and/or (e)auxiliaries and/or additives has been described many times. It ispreferable to avoid adding blowing agents to the starting components forproducing (ii). In order that any uncontrolled foaming process is verysubstantially avoided, the starting components (b) and (c) and also, ifused, (d) and/or (e) should preferably be dry, as should those surfacesof (i) and (iii) which come into contact with the reaction components.

The locations of the openings according to the invention may, forexample, be at points in (i) and (iii) via which the space between (i)and (iii) was filled to produce (ii). For example, appropriate drilledholes in (i) and/or (iii) may be used for this purpose.

The water content of the reaction mixture comprising (a), (b), (c) and,if desired, (d) and/or (e) is preferably from 0 to 0.03% by weight,particularly preferably 0% by weight, based on the weight of thereaction mixture. The appropriate water content, in particular incomponent (b), may be established by distillation, for example. It isalso possible for compounds to be added to the reaction mixture whichbind water and thus prevent any blowing reaction. Compounds of thistype, such as molecular sieves, are well known. Silicates andoxazolidines are examples of compounds which may be used in a suitable,preferably finely divided form. The amounts of the these compoundspreferably added to component (b) are from 0.05 to 5% by weight,particularly preferably from 2 to 4.5% by weight, based on the weight ofthe reaction mixture.

Examples of the starting materials (a), (b), (c), (d) and (e) for thenovel process are given below:

Possible isocyanates (a) are the aliphatic, cycloaliphatic, araliphaticand/or aromatic isocyanates known per se, preferably diisocyanates,which may have been biuretized and/or isocyanuratized by well-knownprocesses. Individual examples are: alkylene diisocyanates having from 4to 12 carbon atoms in the alkylene radical, such as dodecane1,12-diisocyanate, 2-ethyltetramethylene 1,4-diisocyanate,2-methylpentamethylene 1,5-diisocyanate, tetramethylene1,4-diisocyanate, lysin ester diisocyanate (LDI), hexamethylene1,6-diisocyanate (HDI), cyclohexane 1,3- and/or 1,4-diisocyanate,hexahydrotolylene 2,4- and 2,6-diisocyanate, and also the correspondingisomer mixtures, dicyclohexylmethane 4,4′-, 2,2′- and 2,4′-diisocyanate,and also the corresponding isomer mixtures,1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI),tolylene 2,4- and/or 2,6-diisocyanate (TDI), diphenylmethane 4,4′-,2,4′- and/or 2,2′-diisocyanate (MDI), polyphenylpolymethylenepolyisocyanate and/or mixtures comprising at least two of theisocyanates mentioned. Use may also be made in the novel process of di-and/or polyisocyanates containing ester groups, urea groups, allophanategroups, carbodiimide groups, uretdione groups and/or urethane groups.Use is preferably made of 2,4′-, 2,2′- and/or 4,4′-MDI and/or ofpolyphenylpolymethylene polyisocyanates, particularly preferably ofmixtures comprising polyphenylpolymethylene polyisocyanates and at leastone of the MDI isomers.

Examples of compounds (b) which may be used and are reactive towardisocyanates are those in which the group(s) reactive toward isocyanatesis/are hydroxyl, thiol and/or primary and/or secondary amino, forexample polyols selected from the group consisting of polyetherpolyalcohols, polyester polyalcohols, polythioether polyols,hydroxy-containing polyacetals and hydroxyl-containing aliphaticpolycarbonates, and mixtures made from at least two of the polyolsmentioned. These compounds usually have a functionality of from 1 to 8,preferably from 1.5 to 6, particularly preferably from 2 to 6, andpreferably have a molecular weight of from 400 to 8000. They are knownto the skilled worker.

Examples of polyether polyalcohols are those which are obtainable usingknown technology by adding alkylene oxides, such as tetrahydrofuran,propylene 1,3-oxide, butylene 1,2- or 2,3-oxide, styrene oxide andpreferably ethylene oxide and/or propylene 1,2-oxide, to conventionalstarter substances. Examples of starter substances which may be used areknown aliphatic, araliphatic, cycloaliphatic and/or aromatic compoundscontaining at least one, preferably 2 to 4, hydroxyl group(s) and/or atleast one, preferably 2 to 4, amino group(s). Examples of compoundswhich may be used as starter substances are ethanediol, diethyleneglycol, 1,2- and 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-heptanediol, glycerol, trimethylolpropane, neopentylglycol, sugars, such as sucrose, pentaerythritol, sorbitol,ethylenediamine, propanediamine, neopentanediamine,hexamethylenediamine, isophoronediamine,4,4′-diaminodicyclohexylmethane, 2-(ethylamino)ethylamine,3-(methylamino)propylamine, diethylenetriamine, dipropylenetriamineand/or N,N′-bis(3-aminopropyl)ethylenediamine.

The alkylene oxides may be used individually or alternating insuccession, or as mixtures. Preference is given to the use of alkyleneoxides which give primary hydroxyl groups in the polyol. Particularpreference is given to the use of polyols which have been alkoxylatedwith ethylene oxide at the end of the alkoxylation and therefore haveprimary hydroxyl groups.

Suitable polyester polyols may be prepared, for example, from organicdicarboxylic acids having from 2 to 12 carbon atoms, preferablyaliphatic dicarboxylic acids having from 4 to 6 carbon atoms, andpolyhydric alcohols, preferably diols, having from 2 to 12 carbon atoms,preferably from 2 to 6 carbon atoms. The polyester polyols preferablyhave a functionality of from 2 to 4, in particular from 2 to 3, and amolecular weight of from 480 to 3000, preferably from 600 to 2000, inparticular from 600 to 1500.

The novel composite elements are preferably produced using polyetherpolyalcohols as components (b) for the reaction with the isocyanates,advantageously those with an average functionality of from 1 to 8,preferably from 1.5 to 6 and with a molecular weight of from 400 to8000.

The use of polyether polyalcohols offers considerable advantages by wayof improved resistance of the polyisocyanate polyaddition products tohydrolytic cleavage, and due to their lower viscosity, in each casecompared with polyester polyalcohols. The improved resistance tohydrolysis is particularly advantageous for use in ship building. Thelower viscosity of the polyether polyalcohols and of the reactionmixture for producing (ii) comprising the polyether polyalcohols permitssimpler and more rapid filling of the space between (i) and (iii) withthe reaction mixture for producing the composite elements. Low-viscosityliquids give a considerable advantage in shipbuilding since thedimensions, in particular of structural components, are substantial.

Other suitable compounds reactive toward isocyanates are substanceswhich have a hydrocarbon skeleton having from 10 to 40 carbon atoms andfrom 2 to 4 groups reactive toward isocyanates. For the purposes of theinvention, a hydrocarbon skeleton is a succession of carbon atoms whichis uninterrupted and not, as is the case for example with ethers,interrupted by oxygen atoms. Substances of this type which can be used,also referred to below as (b3), are castor oil and derivatives thereof,for example.

Other compounds which are reactive toward isocyanates and which, inaddition to the abovementioned compounds with a usual molecular weightof from 400 to 8000, may be used if desired as chain extenders and/orcrosslinking agents in the novel process are diols and/or triols withmolecular weights of from 60 to <400. It may moreover prove advantageousfor modifying mechanical properties, such as hardness, to add chainextenders, crosslinking agents or, if desired, mixtures of these. Thechain extenders and/or crosslinking agents preferably have a molecularweight of from 60 to 300. Examples of possible compounds are aliphatic,cycloaliphatic and/or araliphatic diols having from 2 to 14 carbonatoms, preferably from 4 to 10 carbon atoms, for example ethyleneglycol, 1,3-propanediol, 1,10-decanediol, o-, m- orp-dihydroxycyclohexane, diethylene glycol, dipropylene glycol andpreferably 1,4-butanediol, 1,6-hexanediol andbis(2-hydr-oxyethyl)hydroquinone, triols, such as 1,2,4- and1,3,5-trihydroxycyclohexane, glycerol and trimethylolpropane,low-molecular-weight polyalkylene oxides containing hydroxyl groups andbased on ethylene oxide and/or on propylene 1,2-oxide and on theabovementioned diols and/or triols as starter molecules and/or diaminessuch as, for example, diethyltoluenediamine and/or3,5-dimethylthio-2,4-toluenediamine.

If chain extenders, crosslinking agents or mixtures thereof are used forpreparing the polyisocyanate polyaddition products, these are usefullyused in amounts of from 0 to 30% by weight, preferably from 1 to 30% byweight, based on the weight of all of the compounds (b) used which arereactive toward isocyanates.

Other compounds which may be used as (b) in order to optimize theprogress of curing during the production of (ii) are aliphatic,araliphatic, cycloaliphatic and/or aromatic carboxylic acids. Examplesof carboxylic acids of this type are formic acid, acetic acid, succinicacid, oxalic acid, malonic acid, glutaric acid, adipic acid, citricacid, benzoic acid, salicylic acid, phenylacetic acid, phthalic acid,toluenesulfonic acid, and derivatives of the acids mentioned, isomers ofthe acids mentioned and any desired mixture of the acids mentioned. Theproportion of these acids by weight may be from 0 to 5% by weight,preferably from 0.2 to 2% by weight, based on the total weight of (b).

(b) is preferably a mixture which comprises:

-   -   (b1) from 40 to 99% by weight of polyether polyalcohol with an        average functionality of from 1.5 to 2.99 and with an average        molecular weight of from 400 to 8000, and    -   (b2) from 1 to 60% by weight of polyether polyalcohol with an        average functionality of from 3 to 5 and with an average        molecular weight of from 150 to 8000, where all of the weight        data are based on the total weight of the mixture.

(b) is particularly preferably a mixture which comprises:

-   -   (b1) from 40 to 98% by weight, preferably from 50 to 80% by        weight, of polyether polyalcohol with an average functionality        of from 1.9 to 3.2, preferably from 2.5 to 3, and with an        average molecular weight of from 2500 to 8000,    -   (b2) from 1 to 30% by weight, preferably from 10 to 25% by        weight, of polyether polyalcohol with an average functionality        of from 1.9 to 3.2, preferably from 2.5 to 3 and with an average        molecular weight of from 150 to 399, and    -   (b3) from 1 to 30% by weight, preferably from 10 to 25% by        weight, of at least one aliphatic, cycloaliphatic and/or        araliphatic diol having from 2 to 14 carbon atoms, preferably        from 4 to 10 carbon atoms, where all of the weight data are        based on the total weight of the mixture.

The weight ratio between polyether polyalcohols and polyesterpolyalcohols in component (b) is preferably >100, particularlypreferably >1000, and in particular no polyester polyalcohols are usedas (b) for producing (ii).

The use of amine-started polyether polyalcohols can also improve thethrough-curing performance of the reaction mixture for producing (ii).The compounds (b), like the other components for producing (ii), arepreferably used with a very low water content, to avoid forming carbondioxide by a reaction of the water with isocyanate groups.

Components (c) used for producing (ii) may be well known compounds,preferably gaseous at 25° C. and a pressure of 1 bar, such as air,carbon dioxide, nitrogen, helium and/or neon. It is preferable to useair. Component (c) is preferably inert toward component (a),particularly preferably toward components (a) and (b), i.e. there ishardly any, and preferably no, detectable reactivity of the gas toward(a) or (b). The use of the gas (c) differs fundamentally from the use ofconventional blowing agents for producing foamed polyurethanes. Whereasconventional blowing agents are used in liquid form and during thereaction either evaporate due to heat generation or else in the case ofwater evolve gaseous carbon dioxide due to reaction with the isocyanategroups, component (c) in the present invention is preferably gaseousbefore it is used.

Shrinkage of (ii) and any resultant partial separation from (i) and/or(iii) is substantially avoided by reacting (a) with (b) in the presenceof (c). Shrinkage of the plastic (ii), for example of the polyisocyanatepolyaddition products, can cause some extent of separation of theplastic (ii) from the metal plates (i) and/or (iii). However, very fulland complete adhesion of the plastic (ii) to the metal plates (i) and/or(iii) is specifically of particular interest for the mechanicalproperties of a composite element of this type.

The catalysts (d) which may be used include well-known compounds whichmarkedly accelerate the reaction of isocyanates with the compoundsreactive toward isocyanates. The total catalyst content used ispreferably from 0.001 to 15% by weight, in particular from 0.05 to 6% byweight, based on the weight of all of the isocyanate-reactive compoundsused. Examples of compounds which may be used are: triethylamine,tributylamine, dimethylbenzylamine, dicyclohexylmethylamine,dimethylcyclohexylamine, N,N,N′,N′-tetramethyldiaminodiethyl ether,bis(dimethylaminopropyl)urea, N-methyl- and/or N-ethylmorpholine,N-cyclohexylmorpholine, N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetramethylbutanediamine,N,N,N′,N′-tetramethyl-1,6-hexanediamine, pentamethyldiethylenetriamine,dimethylpiperazine, N-dimethylaminoethylpiperidine,1,2-dimethylimidazole, 1 -azabicyclo[2.2.0]octane,1,4-diazabicyclo[2.2.2]octane (Dabco) and alkanolamine compounds, suchas triethanolamine, triisopropanolamine, N-methyl- andN-ethyldiethanolamine, dimethylaminoethanol,2-(N,N-dimethylaminoethoxy)ethanol,N,N′,N″-tris(dialkylaminoalkyl)hexahydrotriazines, e.g. N,N′,N″-tris(dimethylaminopropyl)-s-hexahydrotriazine, iron(II) chloride,zinc chloride, lead octoate, and preferably tin salts, such as tindioctoate, diethyltin hexoate, dibutyltin dilaurate and/ordibutyldilauryltin mercaptide,2,3-dimethyl-3,4,5,6-tetrahydropyrimidine, tetraalkylammoniumhydroxides, such as tetramethylammonium hydroxide, alkali metalhydroxides, such as sodium hydroxide, alkali metal alcoholates, such assodium methylate and potassium isopropylate, and/or alkali metal saltsof long-chain fatty acids having from 10 to 20 carbon atoms and, ifdesired, laterally positioned OH groups.

It has proven very advantageous to carry out the production of (ii) inthe presence of (d) in order to accelerate the reaction.

If desired, additives and/or auxiliaries (e) may be incorporated intothe reaction mixture for preparing the polyisocyanate polyadditionproducts (ii). Examples which may be mentioned are surface-activesubstances, fillers, dyes, pigments, flame retardants, agents to protectagainst hydrolysis, and substances with fungistatic or bacteriostaticaction, and the abovementioned molecular sieves, and foam stabilizers.

Examples of possible surface-active substances are those compounds whichserve to promote the homogenization of the starting materials and whereappropriate, are also suitable for regulating the cell structure of theplastics. Examples which may be mentioned are emulsifiers, such as thesodium salts of castor oil sulfates or of fatty acids, and also salts offatty acids with amines, e.g. diethylammonium oleate, diethanolammoniumstearate, diethanolammonium ricinoleate, and salts of sulfonic acids,e.g. the alkali metal or ammonium salts of dodecylbenzene- ordinaphthylmethanedisulfonic acid and ricinoleic acid. The surface-activesubstances are usually used in amounts of from 0.01 to 5% by weight,based on 100% by weight of the total of compounds (b) used which arereactive toward isocyanates.

Examples of suitable flame retardants are tricresyl phosphate,tris(2-chloroethyl) phosphate, tris(2-chloropropyl) phosphate,tris(1,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl) phosphate,tetrakis(2-chloroethyl) ethylenediphosphate, dimethylmethanephosphonate, diethyl diethanolaminomethylphosphonate and alsocommercially available halogen-containing flame-retardant polyols. Thecompounds which may be used to provide flame retardancy to thepolyisocyanate polyaddition products are, besides the abovementionedhalogen-substituted phosphates, inorganic or organic flame retardantssuch as red phosphorus, alumina hydrate, antimony trioxide, arsenicoxide, ammonium polyphosphate and calcium sulfate, expandable graphiteor cyanuric acid derivatives, e.g. melamine, or mixtures of at least twoflame retardants, e.g. ammonium polyphosphates and melamine, and also,if desired, maize starch or ammonium polyphosphate, or melamine andexpandable graphite and/or, if desired, aromatic polyesters. It hasgenerally proven useful to use from 5 to 50% by weight, preferably from5 to 25% by weight, of the flame retardants mentioned, based on theweight of all of the isocyanate-reactive compounds used.

For the purposes of the invention, fillers, in particular reinforcingfillers, are reinforcing agents, weighting agents, agents to improveabrasion performance in paints, coating agents, etc., and the usualorganic and inorganic fillers known per se. Individual examples whichmay be mentioned are: inorganic fillers, such as silicate minerals, forexample phyllosilicates, such as antigorite, serpentine, hornblends,amphiboles, chrysotile and talc, metal oxides, such as kaolin, aluminas,titanium oxides and iron oxides, metal salts, such as chalk, barite andinorganic pigments, such as cadmium sulfide and zinc sulfide, and alsoglass. Preference is given to the use of kaolin (china clay), aluminumsilicate and coprecipitates of barium sulfate and aluminum silicate, andalso to natural and synthetic fiber-forming minerals, such aswollastonite and short metal and glass fibers. Examples of possibleorganic fillers are: carbon, melamine, colophony, cyclopentadienylresins and graft polymers, and also cellulose fibers, polyamide fibers,polyacrylonitrile fibers, polyurethane fibers, and polyester fibersbased on aromatic and/or on aliphatic dicarboxylic esters, and inparticular carbon fibers. The inorganic and organic fillers may be usedindividually or as mixtures.

The auxiliaries (e) and/or additives used in producing (ii) preferablycomprise from 10 to 70% by weight of fillers, based on the weight of(ii). The fillers used preferably comprise talc, kaolin, calciumcarbonate, barite, glass fibers and/or glass microbeads. The sizeselected for the particles in the fillers is preferably such as not toimpede introduction into the space between (i) and (iii) of thecomponents for producing (ii). The fillers particularly preferably haveparticle sizes of <0.5 mm.

It is preferable for the fillers to be used in a mixture with the polyolcomponent in the reaction to prepare the polyisocyanate polyadditionproducts.

The fillers may serve to reduce the coefficient of thermal expansion ofthe polyisocyanate polyaddition products, which is greater than that ofsteel, for example, and thus to match this coefficient to that of thesteel. This is particularly advantageous for a durably strong bondbetween layers (i), (ii) and (iii), since it results in lower stressesbetween the layers when they are subjected to thermal load.

The weight of (ii) by definition corresponds to the weight of thecomponents (a), (b) and (c) and, if desired, (d) and/or (e) used toproduce (ii).

It is preferable for conventional commercially available foamstabilizers well known to the skilled worker to be used as (e) forproducing (ii), for example well known polysiloxane-polyoxyalkyleneblock copolymers, e.g. Tegostab 2219 from Goldschmidt. When producing(ii), the proportion of these foam stabilizers is preferably from 0.001to 10% by weight, particularly preferably from 0.01 to 10% by weight,and in particular from 0.01 to 2% by weight, based on the weight of thecomponents (b), (e) and, if used, (d) used to produce (ii). The use ofthese foam stabilizers stabilizes the preferred component (c) in thereaction mixture for producing (ii).

For the reaction with (a), i.e. for producing (ii), it is particularlypreferable to use a mixture which comprises:

-   -   (b1) from 40 to 98% by weight, preferably from 50 to 80% by        weight, of polyether polyalcohol with an average functionality        of from 1.9 to 3.2, preferably from 2.5 to 3, and with an        average molecular weight of from 2500 to 8000,    -   (b2) from 1 to 30% by weight, preferably from 10 to 25% by        weight, of polyether polyalcohol with an average functionality        of from 1.9 to 3.2, preferably from 2.5 to 3 and with an average        molecular weight of from 150 to 399, and    -   (b3) from 1 to 30% by weight, preferably from 10 to 25% by        weight, of at least one aliphatic, cycloaliphatic and/or        araliphatic diol having from 2 to 14 carbon atoms, preferably        from 4 to 10 carbon atoms,        where all of the weight data for (b1), (b2) and (b3) are based        on the total weight of components (b1), (b2) and (b3),

-   (e1) from 0.001 to 10% by weight, based on the total weight of the    mixture, of foam stabilizers,    and also

-   (e2) from 0.05 to 5 [lacuna], based on the total weight of the    mixture, of molecular sieves.

To prepare the polyisocyanate polyaddition products according to theinvention, the amounts reacted of the isocyanates and of the compoundsreactive toward isocyanates are preferably such that the ratio ofequivalents of NCO groups in the isocyanates to the total of thereactive hydrogen atoms in the compounds reactive toward isocyanates isfrom 0.85 to 1.25:1, preferably from 0.95 to 1.15:1 and in particularfrom 1 to 1.05:1. If (ii) contains at least some isocyanurate groups,the ratio used between NCO groups and the total of the reactive hydrogenatoms is usually from 1.5 to 60:1, preferably from 1.5 to 8:1.

The polyisocyanate polyaddition products are usually prepared by theone-shot process or by the prepolymer process, for example with the aidof high-pressure or low-pressure technology.

It has proven particularly advantageous to operate by the two-componentprocess and to combine the compounds (b) reactive toward isocyanate, thecatalysts (d) if used, and/or auxiliaries and/or additives (e) incomponent (A) and preferably to mix these intimately with one another,and to use the isocyanates (a) as component (B).

The preferred components (c) may be introduced into the reaction mixturecomprising (a), (b) and, if used, (d) and/or (e), and/or into theindividual components described above (a) and (b), or into (A) and/or(B). The component with which (c) is mixed is usually liquid. It ispreferable for the components to be mixed into component (b).

The mixing of the appropriate component with (c) may take place by wellknown processes. For example, (c) may be introduced into the appropriatecomponent by way of well known feeding equipment, such as air-feedingequipment, preferably under pressure, for example from a pressure vesselor compressed by a compressor, e.g. by way of a nozzle. There ispreferably substantial and thorough mixing of the correspondingcomponents with (c), and the size of the bubbles of gaseous (c) in theusually liquid component is therefore preferably from 0.0001 to 10 mm,particularly preferably from 0.0001 to 1 mm.

The content of (c) in the reaction mixture for producing (ii) may bedetermined by way of the density of the reaction mixture using wellknown measurement devices in the return line of the high-pressuremachinery. The content of (c) in the reaction mixture may preferably beregulated automatically on the basis of this density, by way of acontrol unit. Even at very low circulation rates, the component densitycan be determined on-line and regulated during conventional circulationof the material within the machinery.

The sandwich element may, for example, be produced by sealing off,except for a feed and a discharge for the starting components, the spaceto be filled between (i) and (iii) with the starting components forproducing (ii), and charging the starting components (a), (b), (c) and,if desired, (d) and/or (e), preferably mixed, into the space between (i)and (iii), via the feed, preferably using conventional high-pressuremachinery.

The starting components are usually mixed at from 0 to 100° C.,preferably from 20 to 60° C., and, as already described, introduced intothe space between (i) and (iii). They may be mixed mechanically using astirrer or a mixing screw, but preferably by the countercurrent methodusual in high-pressure machinery, in which a jet of A component and ajet of B component, each at high pressure, encounter one another in themixing head and mix. The jet of one or other component may also havebeen divided. The reaction temperature, i.e. the temperature at whichthe reaction takes place, is usually >20° C., preferably from 50 to 150°C.

The polyisocyanate polyaddition products (ii) of the composite elementproduced according to the invention preferably have a modulus ofelasticity of >275 MPa in the range from −45 to +50° C. (in accordancewith DIN 53457), adhesion to (i) and (iii) of >4 MPa (in accordance withDIN 53530), elongation of >30% in the range from −45 to +50° C. (inaccordance with DIN 53504), tensile strength of >20 MPa (in accordancewith DIN 53504) and compressive strength of >20 MPa (in accordance withDIN 53421).

The novel composite elements which can be produced by the novel. processhave the following advantages over known designs:

-   -   The opening according to the invention markedly improves the        performance of the composite element at high pressure        differences and/or at high temperatures.    -   Shrinkage of (ii) and the associated impairment of adhesion        of (ii) to (i) and (iii) can be avoided by the preferred use of        (c).    -   Struts and similar reinforcing elements are almost completely        superfluous. This gives a considerable cost reduction in        manufacturing, via material saving and via significantly simpler        corrosion protection.    -   In shipbuilding applications the lower weight gives higher        tonnage and/or lower fuel consumption.    -   Maintenance, for example in relation to corrosion protection, is        significantly simplified. This gives longer maintenance        intervals.    -   The sandwich structure with the polyisocyanate polyaddition        product, for example with the polyurethane elastomer, gives        better energy absorption and therefore lower crack propagation.        Known steel structures are susceptible to crack formation        following perforation, when a further stress is applied, i.e.        the leak expands to a large area of the hull. This minimizes the        risk of damage in the event of an accident or under exceptional        loads. This improved standard of safety is particularly        advantageous for tankers.    -   Shrinkage can be reduced and the adhesion of (ii) to (i)        and (iii) markedly improved by the use of (c) according to the        invention in producing (ii).    -   The preferred polyisocyanate polyaddition products based on        polyether polyalcohols are more resistant to hydrolytic        degradation than products based on polyester polyalcohols. This        gives considerable advantages, especially for use of the        composite elements in shipbuilding.    -   The preferred reaction mixture comprising the polyether        polyalcohols for producing (ii) has a markedly lower viscosity        than reaction mixtures based on polyester polyalcohols. This        enables more rapid and simpler manufacture of the composite        elements.    -   The preferred content of fillers in the preferred polyisocyanate        polyaddition products reduces the coefficient of thermal        expansion of (ii), and thus brings it closer to the coefficients        of (i) and (iii). Stresses between (i), (ii) and (iii) arising        from thermal stresses, in particular caused by the ambient        temperature, for example in the case of ships' hulls caused by        variations in water temperatures, could be reduced according to        the invention. This gave a lasting improvement in the adhesion        of (ii) to (i) and (iii).    -   The adhesion of (ii) to (i) and (iii) could be markedly improved        by the preferred sand-blasting of the surfaces of (i) and (iii).        Due to the improved adhesion, the structural components obtained        could have better stability and durability.

The composite elements obtainable according to the invention aretherefore applied mainly where there is a need for structural elementswhich withstand large forces, for example as structural elements inshipbuilding, e.g. in hulls, for example double hulls with an outer andan inner wall, and cargo hold covers, or in building works, for examplebridges, or as structural components in the construction of buildings,in particular for multistorey buildings.

The composite elements according to the invention should not be confusedwith traditional sandwich elements which comprise a rigid polyurethanefoam and/or a rigid polyisocyanurate foam as core and are usually usedfor thermal insulation. The comparatively low mechanical load-bearingcapacity of these known sandwich elements would make them unsuitable forthe application sectors mentioned.

1. A composite element which has the following layer structure: (i) from2 to 20 mm of metal, (ii) from 10 to 300 mm of plastic, and (iii) from 2to 20 mm of metal, where (i) and/or (iii) have an opening which may, ifdesired, be sealable.
 2. A composite element as claimed in claim 1,wherein the opening has a diameter of from 0.5 to 100 mm.
 3. A compositeelement as claimed in claim 1, wherein the opening has been closed andopens when the pressure difference between (ii) and the other,outward-facing, side of (i) and/or (ii) is at least 10 bar, where thehigher pressure is present in (ii).
 4. A composite element as claimed inclaim 1, whose opening according to the invention has been closed andopens when the temperature is above 250° C.
 5. A composite element asclaimed in claim 1, wherein the opening is a valve.
 6. A compositeelement as claimed in claim 1, wherein the valve has been connected by amethod using screw threads to (i) and/or (iii).
 7. A composite elementas claimed in claim 1, wherein the opening has been connected to one endof a pipe which extends into (ii) and whose other end is at a distanceof from 0.5 to 9.5 mm from the surface in contact with (ii) of the metallayer which does not have the opening connected to the pipe.
 8. The useof open or closed valves in composite elements which have the followinglayer structure: (i) from 2 to 20 mm of metal, (ii) from 10 to 300 mm ofplastic, and (iii) from 2 to 20 mm of metal, for reducing a pressuredifference between (ii) and the other outward-facing side of (i) and/or(iii), where the higher pressure is present in (ii).
 9. The use of openor closed valves in composite elements which have the following layerstructure: (i) from 2 to 20 mm of metal, (ii) from 10 to 300 mm ofplastic, and (iii) from 2 to 20 mm of metal, for dissipating gases from(ii).
 10. A ship or a bridge comprising composite elements as claimed inany one of claims 1 to 6.